For a nonstop display of flowers, few plants can top the mandevilla vine (Mandevilla spp. or Dipladenia spp.) for colorful blossoms that keep coming all season. If a mandevilla's leaves start turning yellow, however, it's important to identify the cause of the problem and correct it quickly to keep the plant growing well and putting out new flower buds.
A Change in Environment
The mandevilla plant is a woody vine that can grow as a ground cover or in a container with support. It survives outdoors all year in U.S Department of Agriculture plant hardiness zones 9 through 11. If a nursery-grown mandevilla recently transplanted to a garden has yellowing leaves, then it may be experiencing transplant shock, which also can cause the loss of a few leaves. Remove and discard the affected leaves, and give the plant time to recover. It should put out some new growth in a few weeks.
If the yellowing leaves are on a container-grown mandevilla you recently took indoors to overwinter, then the plant might be in shock from the move, which can cause some leaves to droop, turn yellow and fall off. You can minimize the problem by keeping the plant in a sunny spot, such as in front of a south-facing window. Placing its pot on a pebble-filled tray containing water that reaches just below the pebble tops will help raise the area's humidity, reducing the effect of dry indoor air. Remove and destroy all damaged or dropped leaves, and trim back bare stems with sharp shears that you wipe with rubbing alcohol before each cut to prevent spreading plant diseases.
A Cultural Problem
Too Much Water
Leaves that turn completely yellow and then fall from a mandevilla can signal an overwatering problem, especially if the plant grows outdoors and lots of rain occurred lately. If the plant is potted, then allowing it to sit in a water-filled saucer can keep its soil soggy, which causes yellowing leaves. Grow the plant in a pot that has one or more drainage holes in its bottom, and always allow the pot to drain fully after each watering. Whether container-grown or in the ground, give a mandeviilla vine water only when the top 1 to 2 inches of its soil feels dry to your fingertip.
Not Enough Fertilizer
If you see gradual yellowing of all the leaves on a mandevilla plant or if new young leaves have a light-green or yellow-green color, then the plant could be suffering from a lack of nutrients. You can correct this problem by starting a fertilizer regimen, using a water-soluble, balanced formula such as 20-20-20 that you dilute at a rate of 1/2 teaspoon per 1 gallon of water, but also check your fertilizer product's label for its specific instructions. Fertilize the plant every two or three weeks from spring until early fall, using the fertilizer-water solution in place of a regular watering.
If you see white, fluffy spots on a mandevilla's leaves followed by yellowing and wilting of the leaves, then the likely problem is a fungal disorder called powdery mildew. Other fungal diseases such as leaf spot may cause dark, moist-looking spots on leaves that eventually yellow and wilt. If your plant has one of these fungal problems, then remove the affected parts by pruning. Use sharp shears to cut into healthy tissue behind the diseased stems, and dispose of the infected parts right away. Wipe the shears' blade with rubbing alcohol before each cut and after you finish pruning; doing so will help to prevent spreading the disease to healthy tissue.
The best way to prevent the reappearance of these fungal problems is to give the plant good air circulation. Space multiple plants grown as ground cover at least 6 to 8 feet apart, and keep potted plants well-separated. Watering at each plant's base to avoid wetting foliage also helps, as does clearing away plant debris regularly.
A mandevilla plant is susceptible to a disease called southern wilt that is caused by bacteria. The disease causes yellowing and eventual browning of lower leaves, followed by leaf drop. No effective treatment exists; discard and destroy the affected plant. Also, inspect a nursery plant carefully for yellow leaves before purchasing it.
A Pest Problem
A mandevilla plant can attract a few pests that, if uncontrolled, can cause yellowing of leaves and eventual leaf loss. These pests include white, fluffy mealybugs, which are best destroyed by touching each one with a cotton swab dipped in rubbing alcohol, and tiny, white whiteflies, which feed on leaves. Spider mites are not visible to the naked eye, but they cause easily visible weblike coverings on leaves and young growing tips that can turn yellow. You can destroy both spider mites and whiteflies by spraying the affected plant with insecticidal soap, diluted at a rate of 5 tablespoons per 1 gallon of water. Repeat the diluted solution's application every two weeks as needed until you no longer see pests or symptoms of pests.